For the production of integrated circuits in the semiconductor industry, semiconductor substrates or masks, as precursors for the exposure of the semiconductor substrates to light, are coated with photosensitive resists, which are then exposed to light in a photolithographic step with a structure representing a component of the circuit. Depending on the formation of the resist as a positive or negative resist, the exposed or unexposed structures are used, after a development step, as an etch or implantation mask, etc., for transferring the structures to the substrate or the mask.
Owing to the progressive increase in the integration density on the substrates or masks, the sizes of structures to be achieved on the substrates or masks are decreasing. The minimum achievable structure size is related linearly to the wavelength of the light used in the photolithographic step. This means that changeovers to technology generations with reduced structure size often necessitate the provision of exposure apparatuses which operate at shorter wavelength. Thus, in recent years, the changeover from wavelengths of 365 nm through 248 nm to currently 193 nm for the photolithographic structuring was implemented.
A photosensitive resist typically comprises one or more photoactive substances, a coat-forming base polymer and a solvent. The solvent serves for applying the resist to the substrate. The photoactive substance acts on the base polymer on exposure to light in a certain wavelength with a change in its chemical properties. Exposed parts therefore differ chemically from the unexposed parts, so that the desired parts can be dissolved away selectively in a developer process.
For the formation of a mask in the resist, for example for an etching step, it is necessary for the incident light to penetrate the resist coat down to the bottom surface of the resist coat. The resist used must in each case therefore be transparent to the incident light. Moreover, the photoactive substance must also have sufficient sensitivity to the given wavelength of the incident light. In general, however, the resists used in the photolithographic structuring are transparent only over a limited wavelength range. Thus, for example, the diazonaphthoquinone conventionally used as the photoactive substance, in combination with a novolak resin, can be employed only over a wavelength range from about 300 to 450 nm for the incident light.
For formation of structures having widths of 90–110 nm, ArF excimer lasers having a wavelength of 193 nm are currently used. Photosensitive resists, which can be used in the deep ultraviolet (DUV) wavelength range, comprise, as photoactive substance, a photo acid generator which, on absorption of the incident light, liberates an acid that produces an alkali-soluble acid group through acid-catalyzed deblocking of the alkali-insoluble base polymer. This applies, for example, when a positive resist is employed.
The photosensitive resist for the photolithographic structuring at 193 nm comprises a base polymer that is substantially composed of cycloaliphatic parent structures and is transparent precisely at this wavelength. However, the resists still give rise to considerable problems in the transfer of the structure to the substrate. Thus, for example in the case of the exposed and developed structures, roughened edges leading to insufficient passivation of the sidewalls occur to a particular extent. Owing to a reduced stability of the resist walls remaining after the development, they may collapse, for example, also in a subsequent measurement of the structure width in a scanning electron microscope. A further disadvantage arises by virtue of the fact that conventional resists transparent at a wavelength of 193 nm interact chemically with the antireflection coats underneath them, so that resist residues within the tracks between the resist walls may occur.
The use of even thinner resist systems with simultaneous treatment of the substrate with, for example, hard carbon masks has been proposed to date as a solution. However, this leads to a considerably increased process complexity.